When a solution having low concentration and another solution having high concentration are separated by an osmosis membrane (semi-permeable membrane), the solvent of the solution of low concentration permeate through the osmosis membrane to move to the side of the solution having high concentration. An osmotic pressure electric power generation apparatus which generates electric power by rotating the turbine by utilizing this solvent movement phenomenon is known.
There is another type of osmotic pressure electric power generation apparatus, which generates electric power by circulating a working medium within a closed system. For example, an electric power generation apparatus is known, which uses an aqueous solution of ammonium carbonate as a working medium. In this apparatus, the turbine is rotated by water flow created by the difference in osmotic pressure between two types of aqueous solutions of ammonium carbonate having different concentrations from each other. The portions of the ammonium carbonate aqueous solutions used to rotate the turbine are heated for reuse and are separated into gaseous carbon dioxide and gaseous ammonia, and an aqueous solution of ammonium carbonate having a very low concentration. The separated gaseous carbon dioxide and ammonia are reintroduced into water, thus obtaining an aqueous solution of ammonium carbonate having a very high concentration. Therefore, the two types of aqueous solutions of ammonium carbonate having different concentrations thus obtained are re-circulated and used for electric power generation.
Ammonium carbonate has such a good solubility that 100 g of ammonium carbonate dissolves into 100 mL if water at room temperature. With this solubility, it is possible to obtain an osmotic pressure of such a degree that fresh water can be filtered out from sea water (3.5% by weight). Also, ammonium carbonate decomposes at such a low temperature as 60° C. into a gaseous carbonate and gaseous ammonia. An osmotic pressure electric power generation apparatus which uses an ammonium carbonate aqueous solution generates electric power while sending the aqueous solution pressurized by a positive osmotic pressure to its turbine. Here, it is possible to obtain such a high pressure of 250 atmospheres as the osmotic pressure. This is reported as about 10 times high as the pressure involved in the osmotic pressure electric power generation which uses the osmotic pressure of sea water.
On the other hand, in the electric power generation which utilizes the positive osmotic pressure by ammonium carbonate, poisonous and corrosive gaseous ammonia is produced. The production of gaseous ammonia causes degradation within the system and greatly affects the running cost. Further, ammonium carbonate is easy to precipitate, or more specifically, for example, it immediately precipitates at less than 50° C. when used at 6 moles. Therefore, there is a risk in which precipitated crystals may cut the osmosis membrane when the temperature decreases near the membrane. This risk is particularly likely when the maintenance is carried out at room temperature. In order to reduce the risk of the crystal precipitation, it is unavoidable to drive the system at low concentration. As a result, it becomes difficult to obtain a sufficient osmotic pressure.